1. Field of the Invention
The present invention relates to a method of processing a silicon substrate and a method of manufacturing a substrate for use in a liquid ejection head.
2. Description of the Related Art
Japanese Patent Laid-Open No. 2004-148824 describes a method of forming a fluid supply slot of a print head. With the method described in this patent document, first, laser processing or dry etching processing is performed from a back surface of a substrate in which a print head is to be formed, and then wet etching is conducted to a front surface of the substrate to penetrate the substrate. This processing method can reduce the width of a formed fluid supply slot to a minimum size. Accordingly, the number of print heads obtainable in the same wafer can be increased, thus resulting in a cost reduction of print heads.
Japanese Patent Laid-Open No. 2002-176240 describes a method of detecting the progress of laser processing and stopping the processing. With the method described in this patent document, aside from a processing pulse laser beam, a measurement laser beam is used, and a reflection light beam when a laser beam is focused on each of a surface of an insulating resin layer and a surface of a copper layer within a via hole is detected. By the use of the difference between the times when the strengths of the reflection light beams reach their respective peaks, the thickness from the bottom surface of the via hole and the surface of the copper layer is measured. When the measurement value is at or below a specific value, processing using the processing pulse laser beam is stopped. The measurement unit described in this patent document is the technique known as “confocal microscope.”
The confocal microscope is also discussed in “Three-dimensional surface measurement using the confocal scanning microscope,” Applied Physics B 27, pp. 211-213 (1982), Hamilton and Wilson, for example.
Another example unit configured to detect the progress of laser processing is a measurement unit using a femtosecond laser introduced by Aoshima et al. in Proceedings of the 66th Laser Materials Processing Conference of Japan Laser Processing Society.
However, the above-described examples have drawbacks described below.
For laser processing on a slot illustrated in Japanese Patent Laid-Open No. 2004-148824, high-precision control on the depth of the laser processing is a technical issue. Traditionally, the depth of laser processing is set by the time of laser irradiation or the number of pulses. At a processing initial stage where the depth of laser processing is shallow, a relationship between the laser irradiation time or the number of pulses and the depth of processing shows a linearly increasing tendency. However, as the processing continues to progress and the “aspect ratio” defined by the depth-to-width ratio of a slot increases, the linearly increasing tendency is reducing and the depth of processing at the processing surface of a slot section varies. However, because wet etching uniformly dissolves the entire substrate, if a plurality of pilot holes formed in the substrate by laser processing have widely different depths, for example, the slot shape after wet etching largely varies. These variations in the slot shape cause variations in flow resistance of fluid supplied in the slot. As a result, the product performance may greatly vary.
Examples of a unit configured to reduce variations in the depth of laser processing include an apparatus proposed in the above-mentioned Japanese Patent Laid-Open No. 2002-176240 and a measurement unit using a femtosecond laser proposed by Aoshima et al., which is mentioned above. However, for these examples, the direction of incidence of a laser beam and the direction of measurement of the depth of processing are the same (at the same side). Accordingly, the precision of measurement of the depth of laser processing is a problem. For typical processing using pulsed laser light, with laser irradiation, a high-temperature high-density ionized region (hereinafter also referred to as “plume”) or a particulate workpiece material (hereinafter also referred to as “debris”) occurs. Therefore, if laser processing and measurement are simultaneously performed at the same side, measurement light may be continuously affected by reflection or refraction resulting from the plume or debris, and the precision of measurement may decrease. Additionally, in order to enlarge a laser processing area, it is necessary to reduce magnification of condensation of light of an optical system, but such a reduction may also reduce magnification of measurement and decrease the precision of measurement.